widget: fix GtkLabelAccessible NULL links.

widget: fix GtkLabelAccessible NULL links.

Fix testsuite/a11y/about.ui GtkAboutDialog :
"CRITICAL **: atk_hyperlink_get_start_index: assertion 'ATK_IS_HYPERLINK (link)' failed"
That is set widget->priv->accessible as soon as accessible object is generated.

When accessible object is created accessible->priv->widget is set,
if widget->priv->accessible is not , then _gtk_label_accessible_update_links
exits early, thus without creating the links on the accessible side.
(This as it checks for the widget to have the accessible set before proceeding).

https://bugzilla.gnome.org/show_bug.cgi?id=766458